Electric machines, for example, permanent magnet (PM) motors as may be employed in electric power steering systems (EPS) are affected by parameter variations, which impact the overall system performance as a function of temperature, build and changes over life. The motor circuit resistance, R; inductance, L; and the motor torque/voltage constant, Ke; are the three primary parameters, which affect motor control and performance. Over normal operating temperatures, the motor circuit resistance, R changes by up to 60 to 70%; the motor inductance, L varies a modest amount; and the motor constant, Ke varies by as much as +/−7%. In addition, both the motor circuit resistance, R and motor constant, Ke exhibit variations of about +/−5% for build and a degradation of approximately 10% over the life of the system. The motor resistance, R increases with life and the motor constant, Ke decreases over life. On the other hand, build variations of the motor parameters tend to be randomly distributed. Therefore, without some form of temperature, build, or duration/life dependent compensation, the variations in motor output torque and system damping will result in decreased performance of the power steering system.
To account for variations in the resistance R only, a resistance estimation methodology was conceived of and described in pending commonly assigned U.S. patent application Ser. No. 60/154,692, by Sayeed Mir et al. While able to correct for variations in resistance R due to temperature, build, and life, and well suited for its intended purposes, the correction scheme disclosed in that invention was not always capable of addressing varied motor operating conditions. For example, such conditions may include, when the motor is at stall, in quadrant II of the torque vs. velocity plane, at low currents, or at high motor velocities. In a vehicle employing an electronic steering system significant time may be spent at stall and low motor currents (highway driving for example), and large changes in temperature may occur during these periods. Most correction schemes are configured to make corrections very slowly, thus, it may require significant time to eliminate such an error. Of further significance, existing design schemes may not account for variation in other motor parameters, such as motor constant, which may vary significantly over temperature, build, and life.
Steering systems currently exist which employ the use of current controlled motors. In order to maintain the desired current levels as the temperature in the system varies, these current controlled motors are typically equipped with current sensors as part of a hardware current loop. However, there are also other motor designs in existence that, for cost purposes, require the use of a voltage mode controlled system. In such a situation, the same methods described above to compensate for temperature in the current controlled motor cannot be applied to the voltage-controlled motor in a cost effective manner. Without the benefit of numerous expensive sensors, it becomes necessary to obtain accurate estimations of motor resistance R, motor constant Ke, and temperature readings for voltage control, which accurately controls motor torque.